1. Field of the Invention
This invention relates to an anti-skid control system for achieving a satisfactory rotational behavior of wheels of a motor vehicle during a braking operation. More particularly, this invention pertains to an apparatus for measuring a wheel speed for an anti-skid control system, using a pulse signal corresponding to the actual wheel speed and output from a wheel speed sensor.
2. Description of the Related Art
There have heretofore been proposed a variety of anti-skid control systems, wherein during a braking operation, the brake fluid pressure is reduced to prevent the occurrence of an undesirable wheel lock, and the brake fluid pressure thus reduced is then increased to avoid an undesirable extension of the brake stopping distance. Among such conventional anti-skid control systems is one wherein different pressure buildup rates are stored; a desired one of the stored pressure buildup rates is selected; and brake fluid pressure is increased with the selected buildup rate, for example. To effect the pressure buildup rate selection, a system has been proposed, wherein a solenoid-operated valve adapted to be opened and closed with a relatively high frequency is provided in the brake fluid pressure buildup system; a pulse train generator such as a multivibrator is provided in the drive system for the solenoid-operated valve; and the timing with which the solenoid-operated valve is driven by a pulse train signal derived from the multivibrator, is controlled on the basis of a signal representing wheel acceleration (British Patent Specification No. 1305430).
In a conventional anti-skid control system of a motor vehicle, each of the wheels is provided with a wheel speed sensor, which outputs a pulse signal corresponding to the rotational speed of the wheel, and the cycle of the pulse signal is counted to determine the wheel speed. The wheel speed thus measured is read at constant intervals to execute an anti-skid brake control. Therefore, the change of the wheel speed thus measured on the basis of the pulse signal supplied from the wheel speed sensor has a step-like configuration, even though the actual wheel speed is reduced continuously. Particularly, under a low speed running condition, the cycle of the pulse signal for measuring a wheel speed becomes relatively longer, compared with the cycle for executing an anti-skid brake control. In this case, the difference between the measured wheel speed and the actual wheel speed becomes larger, thereby causing an error or delay in an operation to control the brake pressure when executing an anti-skid brake control.